Commercial and military aircraft have stringent requirements for wire harnesses used on board for electrical connectivity to transfer power and electrical signals. Wiring in these applications is subjected to high temperature, fuel vapors, and vibration, all, or any of which, can cause the wire to degrade with time and produce serious and potentially catastrophic failures. Replacing wiring routinely after a certain time is essential, but costly.
One of the sources of wiring degradation is hydrolysis and is a potential problem for any insulation using aromatic polyimide (e.g., Kapton). Kapton, introduced in 1993 and still in current use in wire harness technology in Teflon-Kapton-Teflon (TKT) composite tape, Kapton composites also still have significant performance issues. Kapton-H has been replaced with Teflon-T (Dupont trade name OASIS, U.S. Pat. No. 5,104,966; also see U.S. Pat. No. 6,781,063 B2) which has reduced, but not eliminated the hydrolysis problem.
Hydrolysis is a process by which prolonged exposure to moisture alters the composition of insulation and diminishes its integrity, weakening it to the point where it develops cracks (See FIG. 1). At high temperatures, current can arc between cracks that have developed close together. Arcing causes the insulation to carbonize, which further increases the probability of arcing. Often carbonization is so severe that arcing will set the insulation on fire, and the carbonized length acts like a fuse so that burns propagate down the harness thus compromising entire sections of the harness (See FIG. 2).
Polymer materials are increasingly being used in a wide range of applications in marine and outdoor environments. Often PVC is used as insulation and wire size can vary from 16 AWG (0.050″ diameter) to 8 AWG (0.13″ diameter). Such wire can be used for internal wiring of electrical equipment, internal wiring of panels and meters, and point to point wiring. The wire can be either stranded (cable) or single conductor.
Wiring used in marine applications, such as recreational watercraft, degrades with time from exposure to salt, moisture, and vibration. Over a period of ten to fifteen years, the insulation essentially becomes porous and the metal conductors corrode, often resulting in mechanical failures as well. Another failure mode is the gradual change in surface chemistry from moisture and contamination so that the surface goes from being hydrophobic to hydrophilic. The modified surface then permits the development of electrical arcing that leads to flashover and on-board fires.
Numerous organizations are developing and implementing new testing requirements to measure the flame propagation characteristics of tray cables and the toxic gases generated during combustion of wire and cable used in building plenums. The establishment of a new flame testing criteria for tray cables may eliminate some cable constructions presently permitted in cable tray. Introduction of new materials that either are more flame retardant, produce less smoke, or generate less corrosive or toxic gases will be needed to meet new regulations.
Therefore, there is a widespread need for new wiring insulation constructions that survive for tens of years in high temperature environments, harsh chemical environments, salt-spray environments and environments in which low flammability is a requirement. The solutions must also be cost effective.
Consequently, there is a pressing need for an electrically conducting wire that is provided with a robust insulation that can withstand the rigors of moisture laden environments, salt water environments, and high temperature and vibration environments while remaining highly resistant to abrasion, and it is a principal object of this invention to provide such structures and methods and materials for fabricating them.
Other objects of the invention will be obvious and will appear hereinafter when the following detailed description is read in connection with the accompanying drawings.